1. Field of the Invention
This invention relates to apparatus for detecting ground faults and potentially dangerous current leakage to ground in AC equipment and protecting the user of the AC equipment against the hazards of an electrical shock and more particularly to a ground fault detecting device responsive to a change in Q of a high frequency tuned circuit.
2. Description of the Prior Art
A user of alternating current (AC) electrical appliances could be in danger of an electric shock due to a ground fault or a conductive path for leakage current from a power conductor to a conductor or terminal at ground potential. For this reason, ground fault interrupter circuits have been devised to protect a person from such a hazard. Some prior art ground fault interrupter circuits employ a suitable differential current transformer as a sensing circuit for determining the presence of a ground fault. The differential current transformer has a primary winding and a secondary winding wound on a common core. The primary winding is formed by a first conductor serially coupled between the line terminal of an AC power source and a load, Z.sub.L, and a second conductor serially coupled between a neutral terminal of the AC power source and the load, Z.sub.L. Typically, the neutral terminal of the source is connected to a source terminal at ground potential. Under operating conditions, load currents from the AC power source are substantially confined to the first and second conductors forming the primary winding to provide mutually cancelling magnetic fields in the core, whereby no electrical signal is induced on the secondary winding. However, the presence of a conductive path for leakage current from the first conductor of the primary winding to ground produces an uncancelled magnetic field in the core which in turn induces an electrical signal on the secondary winding. A detector circuit coupled to the secondary winding responds to the secondary winding signal to provide a signal suitable for operating a switching circuit for interrupting power conduction from the AC power source. A problem associated with employing a differential current transformer as a sensing circuit is a decrease in sensing circuit sensitivity in the presence of a conductive path for leakage current from the second conductor to ground.
Other ground fault interrupter circuits are arranged to couple a high frequency supervisory signal to a line conductor and monitor high frequency current conduction in a neutral conductor. A ground fault in the electrical equipment coupled to the line conductor provides an alternate path to ground for the high frequency supervisory signal thus reducing the high frequency current conduction in the neutral conductor. This reduction is sensed by complex circuitry and used to interrupt power conduction to the electrical equipment.
It is desired to produce a ground fault interrupter circuit to interrupt power conduction from an AC source in the presence of a conductive path to ground from either the first or second power conductor without employing a complex circuitry or a differential current transformer.
Referring to FIG. 1, there is shown a block diagram of a prior art ground fault interrupter circuit 10 serially coupled between a load Z.sub.L and an alternating current (AC) source 12 having output terminals 14, 16, and 18. The output terminals 14, 16, and 18 may be included in a typical wall outlet. A conductor 19 connects the source output terminals 16 and 18 to ground G or a conductor at a reference potential. A ground conductor G.sub.1 may be coupled between source terminals 16 and 18 and a casing 21 enclosing the load Z.sub.L. The serial connection of circuit 10 between the source 12 and the load Z.sub.L is provided by coupling circuit 10 input terminals 14 and 16, respectively, and circuit 10 output terminals 27 and 29 to the load Z.sub.L via line and neutral conductors L and N. The ground fault interrupter circuit 10 includes a sensing circuit 20 and a high power switch means 26 arranged to interrupt power conduction from the source 12 in the presence of undesired leakage current. As defined in this specification, leakage current is current conducted along undesired conductive paths from either the line conductor L to ground G or from the neutral conductor N to ground G.
The sensing circuit 20 includes a prior art differential current transformer 28 having a single turn primary winding comprising a first conductor 30 and a second conductor 31 wound on a common core 34. A multiturn secondary winding 32 is also wound on the core 34 and coupled to the high frequency oscillator circuit 22 and the low frequency amplifier circuit 23. The amplifier 23 is a prior art device arranged to provide an output signal in response to a signal at the source frequency. The oscillator 22 is arranged to operate, in the absence of leakage current, to provide a predetermined output signal at a frequency substantially higher than the frequency of the source 12 in response to suitable direct current bias signals from a source, not shown. The function of the oscillator 22 in the overall operation of circuit 10 will be explained below.
The primary winding first conductor 30 is connected between the interrupter circuit input terminal 15 and switch input terminal 39. The primary winding second conductor 31 is connected between the interrupter circuit input terminal 17 and switch input terminal 41. The switch 26 is normally operated in a closed position, in the absence of leakage current to provide a low impedance conductive path from the switch input terminals 39, 41 to the interrupter circuit output terminals 27, 29 in order to conduct power from the source 12 to the load Z.sub.L. In the absence of leakage current, the differential current transformer 28 provides mutually cancelling magnetic fields in the core 34, whereby an electrical signal is prevented from being induced in the secondary winding 32. However, when leakage current causes uncancelled magnetic fields in the core 34, a portion of the source signal is induced in the secondary winding 32 which is in turn coupled to the low frequency amplifier 23. Thus, the differential current transformer 28 may be employed to sense leakage current or a deviation in current conducted by the conductor L relative to the conductor N and provide an output signal at the source frequency on the secondary winding 32. For example, any leakage current from the conductor L to ground G over a current conducting path Y is a current signal not conducted by the conductor N, thereby a portion of the source signal is induced in the secondary winding 32. The magnitude of the signal included in the secondary winding 32 is proportional to the magnitude of the leakage current and the admittance of the path Y. When the magnitude of the secondary winding signal coupled to the low frequency amplifier 23 indicates the presence of a dangerous level of leakage current, the low frequency amplifier 23 responds by providing an output signal suitable for activating the detector means 23 to operate the power switch 26 to interrupt power conduction from the source 12. Thus, if a person's body provided a current conducting path, Y, for leakage current between a poorly insulated portion of the conductor L and ground G, circuit 10 would operate to interrupt power conduction from the source 12.
Normally, the sensitivity of a sensing circuit employing the differential current transformer 28 is decreased in the presence of an undesired conductive path X between the conductor N and ground G since a portion of leakage current conducted along path Y to ground G would be conducted back to the conductor N along the conductive path X. Thus, if conductive paths X and Y were simultaneously present in an appliance, a deviation in current conducted by the conductor L relative to the current conducted by the conductor N would not provide a signal on the secondary winding 32 having a magnitude proportional to the leakage current along path Y. As a result, a potentially dangerous level of leakage current along path Y may not cause operation of the switch 26 to interrupt power conduction to the load Z.sub.L. However, the ground fault interrupter circuit 10 includes the oscillator 22 and detector circuit 24 arranged to detect the presence of a low impedance conductive path X and cause the interruption of power conduction from the source 12 as further described in U.S. Pat. No. 3,879,639 entitled "Ground Fault Interrupters", issued to Richard C. Sircom on Apr. 1, 1975.
The high frequency oscillator 22 has a parallel tuned circuit comprising the impedance of the secondary winding 32 and a capacitor 36 coupled across the secondary winding 32. The parallel tuned circuit 32, 36 is arranged to be resonant at the output frequency of the oscillator 22 and to have a predetermined Q determining the magnitude of an oscillator output signal coupled to the detector 24. The magnitude of the oscillator output signal is substantially determined by the Q of the parallel tuned circuit 32, 36 when the circuit 32, 36 is included in a feedback loop of the oscillator 22. In the absence of the conductive path, X, the magnitude of the oscillator output signal is sufficient to cause the detector 24 to operate the power switch means 26 to conduct signals from the source 12 to the load Z.sub.L. However, the presence of the current conducting path X provides a continuous conductive loop encircling the core 30 and decreasing the Q of the tuned circuit 32, 36 causing a proportionate decrease in oscillator output power. The loop includes conductor N, path X to the ground G, conductor 19 and the connection from terminal 16 to terminal 17 back to conductor N. If the admittance of the conductive path X is sufficient to cause the magnitude of the oscillator output signal coupled to the detector 24 to decrease below a predetermined level, the detector 24 provides an output signal selected to operate the power switch means 26 to interrupt conduction of signals from the source 12.
In summary, the prior art ground fault interrupter 10 includes the differential current transformer 28 arranged to provide a signal at the source frequency in the presence of a conductive path Y from the conductor L to ground G ultimately causing the operation of the switching circuit 26 to interrupt power conduction from the source 12 to the load Z.sub.L. In addition, the ground fault interrupter 10 includes a high frequency oscillator 22 having a parallel tuned circuit comprising the impedance of the winding 32 and the capacitor 26 tuned to be resonant at the frequency of oscillation. The high frequency oscillator 22 provides an output signal having magnitude which is decreased in the presence of the conductive path X from the conductor N to ground G. When the oscillator output signal coupled to the detector 24 is decreased below a predetermined level, the detector 24 causes the switching circuit 26 to interrupt power condition from the source 12.